ahthurungnone :
Okay, so I need to build a server for our small business and am completely new to server processors and the differences. I am on a tight budget though, so I haven't ruled out just using a single processor.
Firstly, what is the advantage of a Xeon or Opteron processor over a quad core consumer processor?
Xeons and Opterons are particularly targeted at using ECC ram and multi-socket configurations. Typical consumer processors could not handle/do those two things. Some say or think that the Xeons or Opterons may be binned with better parts (meaning they should be of better quality which would either translate to longer lifespans or cooler running). Though I haven't seen real evidence to show that Xeons or Opterons could really outlive their consumer counterparts. Also, I think some Xeons have insane amounts of cache compared to their consumer grade counterparts.
If your applications are not that really mission-critical (possible losses of millions of $$ or a project might fail) then you might be able to get away with using consumer grade processors.
Secondly, what is the most important part of the server processor besides core speed: number of cores, FSB speed, L2 or L3 cache size, etc?
Depends on your application, if the app is multi-threaded, then it'd run faster the more cores you throw at it. Some applications benefit with large cache sizes, some have no effect, really depends on the application you're running. You'd probably have greater increase in speed though with an increase in core count or clock speed before cache would become a big factor.
Third, is it worth having multiple processors if only 10 people are accessing the server?
Finally, how much RAM should I use?
Any help is greatly appreciated.
What app are you running, or what is the server for anyway? When you're looking at virtualized OS instances and your clients do some pretty heavy stuff, then more ram and more cores would be better. If you're just looking at a server which would host a small db, or host some apps, then I wouldn't advise spending too much money on it.
On the ram part, if you're allocating for 10 people, then probably somewhere along the lines of 8GB+ or more might be good enough. Though again, it depends on what people are going to do on your server.